The present invention relates to resilient flexible abrading devices of the type sometimes called sanding sponges that each comprise a thin backing layer of foam and includes a layer of abrasive particles adhered together and to the backing layer by a layer of flexible adhesive.
Resilient flexible sanding sponges that comprise a thin backing layer (e.g., about {fraction (3/16)} inch or 0.5 cm thick) of urethane foam and include a layer of abrasive particles adhered together and to the foam backing layer along one of its major surfaces by a layer of flexible adhesive are well known in the prior art. One such sanding sponge is commercially available from Minnesota Mining and Manufacturing Company, St. Paul, Minn. under the trade designation xe2x80x9cSoftback Sanding Spongexe2x80x9d. Typically, a user of that sanding sponge places the surface of the backing layer opposite the abrasive against the palm of his or her hand and rubs the abrasive over a surface to be abraded while conforming the layer of abrasive to the surface being abraded. While such sanding sponges work quite well to abrade objects having parallel surfaces, their backings are too easily torn when they are used to abrade the intersecting surfaces of projections such as the corners of a table top. Hand pressure on the sanding sponge can cause such a projection to penetrate and tear the backing layer of the sanding sponge as it is moved over the projection. With previously used methods for forming the urethane foam backing, the tear strength of that backing has been largely a function of the urethane chemistry used in making the foam. Obtaining a high tear strength for the backing has been difficult, often requiring a great deal of experimentation, experience and luck; and the maximum tear strengths obtained have still been less than those that are desired.
Also, the layer of flexible adhesive in such a sanding sponge that adheres the layer of abrasive particles together and to the foam backing layer along one of it major surfaces must have sufficient integrity and adhesion to the abrasive particles and to the foam backing layer so that it will not break, or release the abrasive particles, or release from the foam backing layer as the resilient flexible sanding sponge is conformed and used to abrade a surface. Such integrity is provided by applying a layer of adhesive material that has a major portion overlaying the major surface of the backing layer to which it is adhered (in which major portion the abrasive particles are imbedded), and only has tentacle like portions extending into the cells open to that major surface of the foam backing layer to attach the adhesive layer to the foam. While such a layer of flexible adhesive material can provide the desired integrity and adhesion to the abrasive particles and to the foam backing layer, its major portion that overlays the major surface of the backing layer is sufficiently thick that it envelopes more of the abrasive particles than might be desired, thereby somewhat restricting the ability of the abrasive particles to abrade a surface over which the sanding sponge is rubbed.
The present invention provides a resilient flexible sanding sponge having a thin backing layer that is about as flexible and conformable as the urethane backing layers on known prior art sanding sponges while providing significantly improved tear strength compared to the backing layers of those prior art sanding sponges.
The sanding sponge according to the present invention includes a foam backing layer having a layer of abrasive particles along one of its major surfaces that are adhered together and to that backing layer by a layer of flexible adhesive material. The backing layer, when measured in accordance with A.S.T.M. test method ASTM D 3574-95, has a compression force deflection of less than 4 pounds per square inch (64 kPa) at 50% deflection (as do prior art sanding sponges) to provide good conformability with a surface being abraded, while having a tear strength of at least 5 pounds per inch (880 Newtons per meter) which gives the sanding sponge significantly improved tear resistance compared to known prior art sanding sponges so that the sanding sponge will not be easily torn when it is used to abrade projections.
That high tear strength backing layer can be provided by using for the backing layer either (1) felted urethane foam; or (2) foam that encompasses a layer of reinforcing material between its major surfaces. Felted urethane foam having a compression ratio of at least 2, and preferably having a compression ratio in the range of about 3 to 4 is useful as the backing layer. Felted urethane foams having compression ratios above 4 could also be useful as the backing layer, however, increasing the compression ratio increases both the stiffness and the cost of the backing layer.
When the backing layer of the sanding sponge is of felted urethane foam it can also encompass a major portion of the layer of flexible adhesive material adhering the layer of abrasive particles together along its surface to provide the needed structural integrity, adhesion to the abrasive particles and adhesion to the foam backing layer that is required of the layer of adhesive material so that it will not break, or release the abrasive particles, or release from the foam backing layer as the sanding sponge is conformed to a surface and used to abrade it, while leaving large portions of the abrasive particles projecting above the layer of adhesive material where they can effectively abrade a surface over which the sanding sponge is rubbed.
Sanding sponges are typically made by coating liquid make coat adhesive over one major surface of the backing layer, coating a layer of the abrasive particles on the adhesive coated surface of the backing layer, and then drying the adhesive. When the backing layer is of felted foam, more of that layer of make coat adhesive will be wicked into and adsorbed in the backing layer than when the backing layer is a layer of non-felted urethane foam, apparently because of the smaller cell size and crushed cell walls of the felted foam. This greater adsorption of the make coat adhesive has several desirable effects. (1) It forms a major structurally sound portion of the layer of flexible adhesive below the surface of the backing layer along which the layer of abrasive particles is adhered (i.e., the portion of the layer of flexible adhesive below the surface of the backing layer is firmly adhered to the cell walls of the foam backing layer and has very few voids that could weaken its structural integrity). (2) Wicking of the make coat adhesive into the felted urethane foam backing material causes the adhesive to draw backing from around the abrasive particles while leaving meniscuses of the adhesive around the abrasive particles to hold them in place, thereby exposing a higher percentage of the abrasive particles for contact with a surface to be abraded than is exposed if less of the make coat adhesive is wicked into the backing layer. Also, (3) wicking of the make coat adhesive into the felted urethane foam backing material appears to cause most of the abrasive particles to become supported closely along the surface of the backing layer (rather than having some abrasive particles supported on portions of the layer of adhesive of different thicknesses as appears to be the case with prior art sanding sponges), thereby providing an outer surface defined by the ends of the abrasive particles opposite the backing layer that is almost as smooth as the surface of the backing layer along which the abrasive particles are adhered, and which appears to be more smooth than the surfaces defined by the tips of the abrasive particles on prior art sanding sponges.